Use of the radio frequency (RF) spectrum is regulated by governmental entities in each country. Typically, a fixed band of the spectrum is allocated to one or more types of primary users. Certain bands of the spectrum may be overloaded, such as cellular network bands, while other bands may be underused, such as television and radio broadcast bands. For example, a town may have only a few television stations or a radio station may cease broadcasting at a certain time of day. Previously, non-licensed users were prevented from using such underused bands of the spectrum.
More recently, regulatory agencies have opened up some bands of the RF spectrum to secondary users, provided such secondary users avoid interference with primary users. Interference is avoided by deferring to primary users when such users are operating and on the portions of a band in which they are operating. Such opportunistic use of under-utilized RF spectrum for wireless communication is often referred to as cognitive radio.
In order to avoid interference with primary users of a band, a secondary user may perform channel sensing on one or more channels of the band to determine whether a primary user is currently using the channels. If such use is sensed, the secondary user may attempt to use other channels or delay its use of the first channels until such use will not interfere. A secondary user may already be using a channel when a cognitive radio attempts to use it or a primary user may begin using a channel while the secondary user is using it. In either case, the secondary user is required to sense the primary user's activity and avoid use of the channel.
Where no use is currently being made of a channel by a collection of intercommunicating cognitive radios (a cell), the radios of the cell may simply receive signals on the channel and use one of several known techniques for determining whether the channel is already in use. Where the cell is already using a channel, the radios of the cell must periodically cease transmitting on the channel to sense use of the channel by a primary user. Such a channel sensing technique may be referred to as quiet period channel sensing.
Alternatively, the radios of a cell may employ channel-hopping: using a subset of the available channels in a band and, at regular intervals, hopping to a different subset, after checking that the new subset is not already in use. In this way, the secondary users will avoid hopping onto channels already in use by primary user and within a predetermined period, hop off of channels that a primary user has begun using. Such hopping techniques may also be used among subcarriers within a single channel.
All these techniques for channel sensing result in the cognitive radios of a cell using less than the full bandwidth available in an RF band. There is therefore a need for a more efficient technique of channel sensing. More particularly, there is a need for channel sensing while continuing to use channels and subcarriers of an RF band.